Polyphenyl compounds, especially diphenyl, diphenyl ether and halogenated derivatives thereof are widely employed as heat transfer fluids. Exemplary of such materials are diphenyl ether or a eutectic mixture of diphenyl ether and diphenyl marketed commercially under the trademarks "Dowtherm" and "Dowtherm A." Polyphenyl compositions suitable as heating media generally exhibit a low vapor pressure, a nearly constant boiling point, excellent heat transfer characteristics and good thermal stability. They find application wherever close process temperature control is a prime requisite.
The heat transfer fluid is typically utilized in a closed recirculating cycle between a central heat source, on the one hand, and one or more heat sinks or process users on the other. The heat sink may be, for example, the reboiler of a fractionating column, a jacketed autoclave, a steam generator and the like. The heat transfer fluid can be used in either the vapor or liquid state. Notwithstanding good thermal stability, however, polyphenyls are nevertheless prone to undergo thermal degradation over a long period of use to form high boiling thermal decomposition products. If allowed to accumulate in the system, the high boiling contaminants would eventually render the entire polyphenyl inventory unfit for further use. Therefore, it is desirable and customary in the art to provide a means for the continuous or semi-continuous purification of the polyphenyl heat transfer fluids. One such system is described in U.S. Pat. No. 3,113,090, relating to the continuous on-line distillation purification of circulating polyphenyl wherein a slip stream of polyphenyls is flashed to a still which is reboiled by a separate stream of polyphenyls.
In carrying out the distillation purification of Dowtherm A or other polyphenyl heat transfer fluid, the impure polyphenyl comprises the feed to the still, purified heat transfer fluid is taken overhead and the bottoms comprises high boiling decomposition products. The high boiling residues are essentially non-volatile. The residue is rejected either continuously or intermittently, preferably when the percentage of high boilers, according to distillation, reaches 50 percent or more. In practice, it has been found that these residues are still flowable when the residue concentration, according to ASTM distillations, is as high as 70 percent.
When the still or purifier is reboiled by condensing vapors of Dowtherm A taken from the vaporizer of the central heat transfer system, as in U.S. Pat. No. 3,575,815, the vapor pressure in the purifier is necessarily less than system pressure by reason of the temperature difference across the reboiler heating coil. If it is desired to return the purified Dowtherm A to the vaporizer in a continuous manner, a vapor compressor or a condenser and condensate return pump must be provided, which equipment adds to the cost and complexity of the unit. In addition, if the temperature level of the heat transfer system is relatively low, i.e., below 600.degree.-625.degree. F., the purifier must be run under vacuum, again increasing capital and operating costs.
In order to allow the operating pressure of the purifier to be established independently of heat transfer system pressure, it is necessary to provide an independent source of reboiler heat for the purifier. One such source is electric resistance heating. Electrically heated vaporizers for Dowtherm A heretofore employed have generally been limited to operation at bulk liquid temperatures not exceeding 600.degree.-650.degree. F. Attempts to extend that temperature upwardly have proven unsuccessful because extreme heating of the liquid film adjacent to the electric heating element leads to rapid thermal degradation of the heat transfer fluid.